Add xmldoc and nullability annotations

osu.Game/Beatmaps/BeatmapDifficultyManager.cs

@@ -317,11 +317,27 @@ namespace osu.Game.Beatmaps
 
     public readonly struct StarDifficulty
     {
+        /// <summary>
+        /// The star difficulty rating for the given beatmap.
+        /// </summary>
         public readonly double Stars;
+
+        /// <summary>
+        /// The maximum combo achievable on the given beatmap.
+        /// </summary>
         public readonly int MaxCombo;
 
+        /// <summary>
+        /// The difficulty attributes computed for the given beatmap.
+        /// Might not be available if the star difficulty is associated with a beatmap that's not locally available.
+        /// </summary>
+        [CanBeNull]
         public readonly DifficultyAttributes Attributes;
 
+        /// <summary>
+        /// Creates a <see cref="StarDifficulty"/> structure based on <see cref="DifficultyAttributes"/> computed
+        /// by a <see cref="DifficultyCalculator"/>.
+        /// </summary>
         public StarDifficulty([NotNull] DifficultyAttributes attributes)
         {
             Stars = attributes.StarRating;
@@ -330,6 +346,10 @@ namespace osu.Game.Beatmaps
             // Todo: Add more members (BeatmapInfo.DifficultyRating? Attributes? Etc...)
         }
 
+        /// <summary>
+        /// Creates a <see cref="StarDifficulty"/> structure with a pre-populated star difficulty and max combo
+        /// in scenarios where computing <see cref="DifficultyAttributes"/> is not feasible (i.e. when working with online sources).
+        /// </summary>
         public StarDifficulty(double starDifficulty, int maxCombo)
         {
             Stars = starDifficulty;